Laser having two modes at different frequencies

Abstract

A laser for use in all fields of measurement using a heterodyne effect delivers two waves at different frequencies, having mutually orthogonal linear polarizations. It comprises a lasing medium placed in a laser resonating cavity defined by mirrors and a birefringent crystal placed on the path of the two waves. The lasing medium is located in the resonating cavity in a zone where the two waves are separated spatially. Two quarter wave plates, one of which has neutral lines at substantially 45° from the polarization axes of the birefringent means, are placed in the common path of the two waves so as to give a helical polarization to the two waves between the plates.

We claim: 
     
       1. Laser for delivery of two waves having different frequencies and mutually orthogonal linear polarization directions, comprising: a laser resonating cavity defined by two mirrors;   a lasing medium located in only a portion of said laser resonating cavity for being traversed by light reflected between said two mirrors;   birefringent means located outside said portion and traversed by said light for separating said light into two waves having parallel paths when said light is in a first direction and recombining the light from said two paths into a common path when said light is in an opposite direction, said portion being so located with respect to said birefringent means that the light is separated in two parallel paths throughout said portion; and   two quarter-wave plates located for being traversed by said light along said two paths, one of said quarter wave plates having neutral lines at approximately 45° from the polarization axes of the birefringent means, whereby the two waves are helically polarized between the two quarter-wave plates.   
     
     
       2. Laser according to claim 1, wherein said quarter-wave plates and said birefringent means are placed so that the paths of the two waves are separated spatially throughout the whole laser resonating cavity. 
     
     
       3. Laser according to claim 1, wherein said one of said quarter waves is closer to the birefringent means than the other of said quarter wave plates and on a same side of said birefringent means. 
     
     
       4. Laser according to claim 3, wherein said other of said quarter wave plates is angularly adjustable. 
     
     
       5. Laser according to claim 4, further comprising means for automatically controlling the angular position of said other of said quarter wave plates for maintaining the frequency difference between the two waves at a predetermined value. 
     
     
       6. Laser according to claim 5, wherein said control means comprise means for beating the two waves, means for measuring the beat frequency between the two waves and means for controlling the angular position of said other of said quarter wave plates for rendering said beat frequency equal to a reference frequency. 
     
     
       7. Laser according to claim 1, wherein said lasing medium is located between said birefringent means and additional birefringent means. 
     
     
       8. Laser according to claim 1, wherein said birefringent means is a crystal. 
     
     
       9. Laser according to claim 1, wherein the other one of said quarter wave plates, said one of said quarter wave plates, a birefringent crystal constituting said birefringent means and said lasing medium are placed in series relation between a spherically concave mirror and a partially transparent flat mirror.